Lasers, especially those used in tactical military equipment, are subject to a wide range of ambient temperatures which can adversely affect their operation. Optimum laser operation and power output require that the laser resonator or cavity have a length or spacing between the mirrors thereof which is an integral number of half wavelengths of the operating frequency. Since laser resonators are often as much as 50 cm. in length, a resonator which is not temperature compensated will expand through numerous half wavelengths for each 1.degree. C. temperature rise, even at the longer infra red wavelengths.
One solution to this problem has been to provide a thermally compensated resonator structure so that the resonator length remains constant regardless of temperature changes caused either by heat generated by the laser itself or due to ambient temperature effects. Such a resonator is much more expensive, heavier and less rigid than one which is not thermally compensated.
The approach of the present invention involves using a nonthermally compensated conventional laser resonator, with its simplicity, rigidity and other advantages, and to provide a special frequency control system for the laser which permits continuous operation during warmup despite the fact that the laser cavity is scanning or expanding through a large number of longitudinal orders during this time. The frequency control system includes a resonator length-controlling transducer to which one of the laser mirrors is attached. An electrical signal applied to the transducer will thus move the mirror to compensate for thermally induced resonator length changes.
Transducers for such applications normally have a maximum extension of approximately 15 microns which is only about 11/2 wavelengths of the output of a CO.sub.2 laser. Thus such a transducer would normally only be capable of compensating for 15 microns of resonator length change, but, as stated above, a long thermally uncompensated resonator might expand more than this for a 1 degree temperature rise. The present invention provides a feedback frequency control system which automatically senses when the transducer and its attached mirror are running out of extension and in response thereto automatically switches the transducer to another mode or order which is near the mid-range of the transducer's extension. This effectively extends the transducer's length severalfold. This mode transition simply involves rapidly moving the transducer and mirror a distance equal to an integral multiple of a half wavelength of the operating frequency. Since ceramic peizoelectric tranducers (PZT) as well as magnetic transducers of the magnetostriction type have hysteresis and and non-linearities which are both small and repeatable, it has been found experimentally that the delta voltage required to re-step the transducer by one order (or one half a wavelength) varies less than 2% over a wide range. This variation is known as the mode transition error, and it will cause a small temporary frequency error in the laser output which will be corrected by the frequency control system within a millisecond or so. The plus and minus delta voltages are inserted in the frequency control loop at a point after the low pass filter therein which determines the control loop frequency response, and thus the delta voltage will be applied to the transducer to accomplish the mode transition before the frequency control loop can respond thereto.